Damage characterization of belt conveyor made of the 330Nb alloy after service in a carburizing atmosphere in a continuous heat treatment furnace

Abstract In this paper, the failure of the belt conveyor made of 330 Nb heat-resistant steel, working in a furnace for continuous heat treatment in a carburizing and oxidizing atmosphere is investigated. On the basis of microscopic tests and X-ray phase analysis, changes in the microstructure in the cross-section of the conveyor wire after 731, 1642 and 2138 cycles carburizing and oxidizing in the temperature range 30–850 °C in 160 min were determined. Moreover, tensile tests were carried out to investigate the effect of the microstructure on the mechanical properties of heat-resistant steel. The failure was due to a cross crack of wires of the conveyor belt mesh. The results show that the on the surface of the alloy, Cr2O3 chromium oxides and FeCr2O4, Mn1.5Cr1.5O4 oxides are formed. On the border of the alloy and chromium oxides, the airtight SiO2 layer is created. As a result of the actions of external forces, transverse and longitudinal cracks are formed in the oxide layer. These cracks are the ways of carbon and oxygen diffusion. In addition, in the oxidation and carburization process, degradation of the SiO2 layer is promoted by the presence of Na, which is applied in washing agents used for cleaning products before placing them on the conveyor. Microstructure of the unused wire of the conveyor belt consisted of G and H phases and NbC in austenitic matrix. Under the operating conditions of the conveyor belt, phase H dissolves in the matrix with the simultaneous increase of the G phase, and also the G phase is formed around the NbC carbides. High temperature carburizing leads to the precipitation of massive carbides M23C6 with simultaneous significant depletion of chromium into an austenitic matrix to about 13%at. Quantitative microstructural analysis showed that the there was an increase in the size and participation, of hard phases M23C6, G, NbC in the alloy microstructure. The volume fraction of precipitation increased from 3 to 24%vol. The resulting structure is characterized by low strength. The ductility of the alloy has decreased significantly.